Over the years we have seen prep design and the approach to tooth reduction change. With the rise of full contour zirconia, this has resulted in both benefits and challenges.
Zirconia popularity is driven by the fact that it is extremely strong and allows for a more cost-effective restorative option. The offsetting challenges are insufficient crown reduction and esthetics.
Zirconia originally was designed to be covered with layering material much like a PFM. The layering material gave the technician an opportunity to hide the reflective properties of the zirconia. The down- fall was that the layering material was a tenth of the strength of the core causing chipping and breakage. The core also had to be designed at minimum thickness, not taking advantage of the super strength of the material.
Full contour monolithic zirconia crowns originally came out as a super tough functional crown. They tended to not look great but were cost effective and minimized breakage by maximizing the core material, therefore eliminating the layering.
Years later, the lab industry has learned to handle the zirconia. The results allow it to look better by staining the raw, pre-sintered material. Also, new hybrid blended materials have been introduced, allowing for much better aesthetics. New materials are being introduced all the time which are looking nicer and nicer. Some of the highest aesthetics cases in the lab today have little or no layering at all, the result being the best of both worlds; strength and aesthetics.
Manufacturers recommendations stating material thickness minimums can be as thin as .6 mm, has created challenges and concerns for long term successes of these restorations. Although these thicknesses seem to be enough, dentist’s philosophies, in many instances, have shifted to extremely minimal reduction of tooth structure for a crown prep. Although this is good for the over all health of the tooth, it is causing the industry to under prep teeth, resulting in very thin zirconia crowns. When dentists approach prep design with minimum thickness values as the goal, this does not allow for any crown adjustments without compromising the restoration. In fact, there are an increasing number of preps that require a reduction coping just to get the minimum thickness requirements. The long-term success of these crowns may be fine, however, in my mouth I would wish to have extra material thickness to allow for these factors.
Over the years we have seen teeth prepped with enough room for PFM crowns and eMax, but now with FCZ, more preps than ever are under reduced. My worry is that the minimum is not enough. I feel over engineering and allowing a little more tooth reduction when possible, give us better long-term results. These reductions will still be less than previous material requirements. We need to approach prep design and crown design with minimal material thickness not being the goal.
Reference: Zirlux Zirconia Preparation Guidelines
Jamie Schell, RDT
Owner, Schell Dental Ceramics